You must play this as "Multiplayer - Custom game" otherwise you wont get the achievement. So selecting "Single Player - Versus AI" is actually a waste of time when you want achievements i think. I always create a custom game in MP and have no trouble getting achievements. As for the map/game settings, "melee" works for me. Good luck!
For 2v2:
For 2v2, check out these
For 3v3, there are existing articles out there:
I guess that once you're good at the above the 4v4 doesn't differ much, I wouldn't suggest playing it either as you will need to find more people and will have to wait longer. It isn't competitive either, as Tzenes has pointed out earlier in one of his answers. The maps aren't made for it which can make the choice of races unbalanced...
unit synergy of different races
I would suggest every race to create his own composition of units, why?
If you create only one type of units instead and your friend the others,
you will depend much more heavy on one resource. (Example: Marines would be minerals only)
When controlling your army, if you are alone it's much more easier to command your different types of units so they don't accidently die. Then you avoid sending in vikings over a group of enemy marines... If you were separated, you would have to share such thoughts with your friends and then his vikings might arrive to early or too late.
When talking about different races, you can ask and plan what units to create to accompany your friends' army. If he's planning an air assault, you could create some ground units just to distract the opponent away from the air as you both attack...
is there some kind of roles in good teams?
This could be done, but can be risky depending on how you do it...
For example, if you have a player that's only harvesting, his defense/army would probably be weaker and thus you will need to defend his base to counter that. But by doing that, you weaken your own base and have less units where you actually would want them...
So, for that role for example, don't just point one out as the harvester but also plan for his harvesting to not fail as you will be dependent on it. Think of his base as one of your expansions. This is probably an early-game problem to think about, maybe he initially should create an army to defend against people rushing him...
Best Answer
For every outmatched achievement, I did the exact same thing:
Only do the Insane matches (The VHard Hard and Medium Achievements come along with the Insane one)
Play on Megatron, it offers a great base that you can hit every enemy from one central position, which leads me to..
Play Protos v Terran. Do a standard Cannon Rush: Rush one of your initial probes to the center of their 4 base area.
Build a few probes while the rusher is in transit. As soon as he his his location you should have enough minerals to build your first pylon.
The key is to build it far enough from all bases to not bee seen when placing. It cannot be seen when its initially placed, or they will send SCV's at you. If they see it while its building, thats no big deal. They just can't see the initial warp.
Wait for the Pylon to complete, then throw down a forge. Wait for the forge, then start placing Photon Cannons at the perimeter of your lone pylon's power range.
Rinse and repeat. Just keep placing cannons and pylons to expand as needed. Eventually they will quit as you've damaged their game too much.
Warnings
If you don't damage them enough with the initial few cannons, they will ramp into Mech and you will lose.
If they see you place the forge or the pylon, they will mass attack with SCV's and you lose.
Follow those steps and you will be on your way to Outmatched Crusher